This invention relates in general to techniques for forming molded plastic packages with integrated heat sinks and in particular, to a technique for forming molded plastic packages with integrated heat sinks that prevents "flash" from forming on external surfaces of the heat sinks during the molding process.
Molded plastic packages with integrated heat sinks are disclosed, for example, in U.S. Pat. Nos. 3,930,114 and 5,041,902, both of which are incorporated herein by reference. An integrated heat sink (or heat spreader) is molded (i.e., integrated) in a molded plastic package for the purpose of dissipating heat away from an integrated circuit die encased in the plastic package. The integrated heat sink does this by having an internal surface thermally connected to the integrated circuit die and an external surface exposed through an exterior surface of the molded plastic package.
One problem encountered in the fabrication process for forming plastic packages with integrated heat sinks, is the problem of excess mold material or "flash" creeping around the heat sink during the molding process. Such "flash" can either entirely or partially cover the external surface of the heat sink which is to be exposed through the exterior surface of the molded plastic package.
Conventional techniques attempt to prevent formation of such mold "flash" by causing, through various means, the external surface of the heat sink to be pushed up against an internal surface of the mold. Such conventional techniques, however, are not always successful. One reason, for example, is because one of the pushed together surfaces, i.e., that of the heat sink or the mold, may not be planar, thus allowing mold "flash" to creep in and subsequently, solidify between the two surfaces. Such nonplanar surfaces become increasingly more likely as package dimensions increase, and the size of the heat sinks integrated in those packages increase accordingly.
Since the presence of such mold "flash" inhibits the desired heat transfer from the integrated circuit die to the exterior of the molded plastic package, it is desirable to remove it. Removal of such mold "flash," however, can be time-consuming and difficult, requiring an additional fabrication step after the molded package is released from the mold.
For example, mechanical "deflashing" techniques remove such mold "flash" by grinding it away (along with portions of the exterior surface of the molded plastic package through which the external surface of the heat sink is to be exposed), and chemical "deflashing" techniques remove such mold "flash" by dissolving it away (along with portions of the exterior surface of the molded plastic package), e.g., by immersing the molded plastic package in a nitride acid. One such mechanical "deflashing" technique is to blast the exterior surface of the molded plastic package with a fine sand or ceramic powder, and one such chemical "deflashing" technique is to immerse the molded plastic package in a bath of nitride solution. Such mechanical and chemical "deflashing" techniques, however, are both time consuming and expensive, because they require additional equipment, fixtures, and chemicals.